Can I insert the attribute frameborder="0" with javascript? If so, how? I've been fiddling with this method for a couple of hours, but have not succeeded so far. The full(y adjusted) HTML + JS code would be appreciated, to save discussion time.

Just because the validator can't check the attributes that are added via JavaScript doesn't mean that they are valid when added that way. If the attribute is invalid when in the HTML in the first place then it is still invalid when added from JavaScript. The only difference isa that by adding it via JavaScript you are lying to the validator about what the HTML contains.

I tried your code in Chrome and IE9, and it works correctly. But in IE8, however, the frame border still shows. In order to support IE8, it would seem, you'll have to use the attribute. If the validator balks, remember that the validator is a tool to help you find errors. If you know that the frameborder attribute is intentional and not an error, then feel free to ignore that message from the validator.